Nanomechanics of Surface Modified Nanohydroxyapatite Particulates Used in Biomaterials

Hydroxyapatite is an important constituent of natural bone, and possesses excellent biocompatibility and bioactivity, but its brittle nature limits its use for bone tissue engineering. Nanohydroxyapatite (nanoHAP) has been used in synthesis of biomimetic composites for more than a decade, yet the mechanics of nanoHAP particles is not fully understood. The present work attempts to advance the current understanding of mechanics of hydroxyapatite at nanoscale, by carrying out systematic nanoindentation experiments on nanoHAP and surface modified nanoHAP [prepared by in situ mineralization in presence of polyacrylic acid (PAAc)]. Quantitative nanomodulus maps of both modified and unmodified HAP nanoparticles indicate that various surface features of HAP nanoparticles can be probed. Dips in values of elastic moduli across the nanoparticle surfaces in modified nanohydroxyapatite are indicative of composite responses from both polymer and mineral phases (PAAc-HAP) on the surface. Nanoindentation experiments were performed at 100, 1,000, 3,000, 5,000, and 8,000 mu N loads, respectively, to obtain the indentation response from both shallow and deep penetration depths. Nanoindentation results at shallow penetration depths are influenced by nanoscale surface roughness of irregular-shaped HAP nanoparticles and nonuniform distribution of PAAc in the microstructure. Significant nonbonded interactions between HAP and PAAc, as well as the mechanical properties of individual constituents (HAP and PAAc) lead to superior nanomechanical properties of surface-modified nanoHAP as compared to unmodified HAP. The overall inelastic nanomechanical response (including damage leading to reduced overall elastic modulus) is strongly influenced by the nature of the interfaces between the nanoparticles, especially when indent size is much larger than the particle size.